The present invention relates to a method for measuring temperature in a wide range, using a tunnel junction, in which the tunnel junction comprises two metallic conductors with a thin insulation between them, which has a measurable resistance.
A thermometer based on a Coulomb blockade is known from Finnish patent number 102695 (corresponding U.S. Pat. No. 5,947,601). The phenomenon known as a Coulomb blockade causes a conductance drop at the zero point of the bias voltage, the characteristic magnitudes of which depend on the temperature. However, such a conductance drop can only be detected at low temperatures, the method being difficult to apply at temperatures above 20 K. The aforementioned patent discloses achieving a measurement range of 1-100 K with the aid of the smallest size of tunnel junction, but in practice this is extremely difficult to achieve. Thus a related technique has been sought, by means of which it is possible to reach as high as to the room-temperature range.
The present invention provides a method for measuring temperature T in a wide range using a tunnel junction which includes two metallic conductors and a thin insulator between the conductors, which has a measurable resistance R. The method is characterized in that the resistance R can be measured over the linear section of the voltage-current curve and the temperature T determined from the formula:             1      R        =                  1                  R          0                    ⁢              (                  1          +                                    (                              T                                  T                  0                                            )                        2                          )              ,
in which R0 is a previously calibrated constant and T0 is a material constant. According to the invention, the insulating material is an oxide of one conducting material and at least one conductor is fabricated from a material, which belongs to the group aluminium (Al), chromium (Cr), niobium (Nb), copper (Cu).
The measurement is carried out by using one chain of several tunnel junctions or several such chains connected in parallel.
The method is further characterized in that the measured resistance R is fit in the range 10 kxcexa9-1000 kxcexa9. The measurement is carried out using low-frequency alternating current, the amplitude of which is in the range 0.1-10 mV for each of the tunnel junctions in the chain.
The measurement may be calibrated by cooling the tunnel junction being measured to a very low temperature, in which case the constant R0 being calibrated is obtained straight from the measured resistance R with a high accuracy.
The invention is largely based on the realization that the relative change in the conductance of a tunnel junction is proportional to the square of the absolute temperature and that the proportionality coefficient is independent of the thickness of the insulation of the tunnel junction. Although, according to the previously known theory, a square-law dependence was expected, a dependence on the thickness of the insulation was also predicted, which, according to observations, is not the case. The method, according to the invention, for measuring temperature requires only a single calibration point, which is possible to obtain by simply cooling the sensor to a very low temperature. The other benefits and applications of the invention are disclosed below, in connection with the embodiments.
In the following, the invention is examined with reference to s the accompanying drawings which show the principles and embodiments of the method according to the invention.